1. Field of the Invention
This invention relates generally to the art of thin film device processing and fabrication. More specifically, the invention relates to the fabrication of Organic Light Emitting Diode devices and displays.
2. Related Art
Display and lighting systems based on LEDs (Light Emitting Diodes) have a variety of applications. Such display and lighting systems are designed by arranging a plurality of photo-electronic elements (“elements”) such as arrays of individual LEDs. LEDs that are based upon semiconductor technology have traditionally used inorganic materials, but recently, the organic LED (“OLED”) has come into vogue for certain lighting and display applications. Examples of other elements/devices using organic materials include organic solar cells, organic transistors, organic detectors, biochips, and organic lasers.
An OLED is typically comprised of two or more thin at least partially conducting organic layers (e.g., an anode buffer layer (ABL) which transports holes and an emissive layer (EL) which emits light upon hole-electron recombination therein) which are sandwiched between two electrodes, an anode and a cathode. Under an applied potential, the anode injects holes into the ABL which then transports them to the EL, while the cathode injects electrons directly to the EL. The injected holes and electrons each migrate toward the oppositely charged electrode and recombine to form exciton in the EL. The exciton relaxes to a lower energy state by emission of radiation and in process, emits light. The anode is usually fabricated over a substrate. In a bottom-emitting OLED, the substrate is made transparent or nearly transparent to allow light to output from the device.
The light output of OLED devices can be fine-tuned by the use of micro-cavity structures such as a DBR (Distributed Bragg Reflector). The DBR consists of a stack of sub-layers, with each sub-layer having a different refractive index than its adjacent neighbors. The DBR stack also provides internal reflectivity of varying degree depending upon the length of the DBR stack, the materials used, and the angles of incidence of light into those materials. In a bottom-emitting OLED device, the DBR stack is typically designed to be between the substrate and the anode. Typically, a DBR stack designed to enhance a specific wavelength will amplify light at that resonant wavelength resulting in a narrowing of the spectral emission (the bandwidth) and increase in luminance at the resonant wavelength. Additional effects of a DBR include a focusing of light in to the forward viewing angle cone, thus reducing losses due to trapped light in the substrate by improving the out-coupling of light. Other methods that have been used to improve the out-coupling of light are surface modifications on the outside of the substrate glass which are specifically designed to reduce internal reflections, such as lenses. However, since the DBR already channels light into the forward viewing angle effectively, the addition of lenses on the outside of the substrate provides no extra benefit.
Thus, there is a need for an OLED device structure where the light out-coupling can be further improved.